Portable and cordless electric devices are very widely used nowadays. Owing to the continued miniaturization of these portable electronic devices, the demand for increasingly small and increasingly light secondary batteries having a high energy density, which serve as an energy source for such devices, has very rapidly increased in recent years. Secondary batteries used are mainly nickel metal hydride batteries as well as lithium ion batteries. In consumer applications (e.g. mobile phone, laptop, digital camera), virtually exclusively only lithium ion secondary batteries occur since they have a substantially higher energy density compared with the nickel metal hydride batteries.
This type of secondary battery is distinguished by active materials on the cathode and anode side which can reversibly incorporate and release lithium ions. When this battery type was launched in the early 90s, lithium cobalt oxide LiCoO2 was used as the electrochemically active substance for the positive electrode. However, this LiCoO2 which currently still dominates the market for active cathode materials in lithium ion secondary batteries, has a disadvantage of a very high cobalt price and greatly limited availability of cobalt. Against a background of the greatly expanding markets for Li ion technology (i.e. power tools, hybrid engine vehicles (HEV) as new applications), the limited availability of cobalt gives cause for concern that LiCoO2 alone will not be able in future to supply the market for active cathode materials for Li ion batteries. Even at present, more than 25% of the annual cobalt production is used in the battery sector. Alternative cathode active materials are therefore urgently necessary.
Inter alia, against this background, the use of LiNiO2 as active cathode material for Li ion batteries has already been discussed for a relatively long time. Nickel is both substantially more economical than cobalt and available in much larger amounts. In addition, LiNiO2 has a substantially higher electrochemical capacity than LiCoO2.
However, such an LiNiO2 has the disadvantage that, when used in secondary batteries, it leads to insufficient thermal stability of the battery. A significant change in the crystal structure during the charging/discharging process furthermore means that the long-life properties/cycle stability of the batteries with such an active material does not meet the market requirements.
For improving the abovementioned parameters, various doping elements for LiNiO2, such as, for example, Co, Al, Mn, Fe and Mg, were therefore tested over the years, which led to a significant improvement in the parameters discussed above. Example compounds having the dopants mentioned are LiNi0.80Co0.15Al0.05O2 and LiNi0.33Co0.33Mn0.33O2. These dopants permitted the market launch of the nickel-containing lithium mixed metal compounds, which are currently used in addition to the original active material, LiCoO2.
In the case of the required, high-energy density of the storage media (secondary batteries), a distinction may be made between the volumetric energy density, expressed in watt hours/liters (Wh/l), and the gravimetric energy density, expressed in Wh/kg. The volumetric energy density of the secondary battery is influenced, inter alia, by the electrode density (g/cm3) both on the side of the cathode and on the side of the anode. The higher the electrode density of the cathode or anode, the higher is the volumetric energy density of the storage medium. The electrode density in turn is influenced both by the production process of the electrodes and by the active cathode material used. The higher the density of the cathode material (for example, determined as tapped density, compacted density or compressed density), the higher is the resultant electrode density under otherwise constant conditions during electrode manufacture (e.g. processes for electrode manufacture, electrode composition). This discovery is already reflected in some documents.
Thus, DE 19849343 A1 describes the compacted density of lithium-containing mixed oxides of the formula LiNiCoMO2. Here, M is at least one of the metal elements Al, Ca, Mg and/or B. The compacted densities of these materials, the primary particles of which have rectangular or square structure, and the secondary particles of which are spherical, are in the range of 2.4 to 3.2 g/cm3.
In DE 19849343 A1 it is pointed out that the morphology and particle shape of the precursor are of major importance for the shape of the product (the LiNiCoMO2) and hence also the compacted density thereof. Furthermore, it is stated that a higher compacted density increases the relative packing quantity of an active material for a positive electrode, with the result that the capacitance of an electrochemical cell is increased. The importance of spherical particles for achieving high compacted densities is also mentioned.
The relationship between tapped density of the active cathode material and electrode density and hence volumetric energy density of the Li ion battery is described in Journal of The Electrochemical Society, Vol. 15 (2004), 10, pages A1749-A1754.
Since a certain pressure is applied during the electrode preparation, the tapped density or compacted density determined for the powder need not, however, permit direct conclusions about the electrode density when this powder is used. A compressed density of a powder which is determined under a defined pressure represents a variable which permits more reliable conclusions about the electrode density with this powder. A precondition for the abovementioned measurement of the compressed density as well as for the electrode manufacture should be that the particles do not break during the compression. Breaking of the particles would mean firstly uncontrolled manufacture of the electrode and furthermore such comminution of the particles during the electrode production would lead to inhomogeneities. Thus, the internal fracture surfaces of the comminuted particles would not have such good contact with the binder and the conductive additive of the electrode as the external surface of the particles. US 2004/023113 A1 is concerned with the determination of the compressed density and compressive strength of cathode powders.
Substances of the general formula LixM(1-y)NyO2 in which 0.2≦x≦1.2, 0≦y≦0.7, are mentioned therein. Here, M is a transition metal and N is a transition metal differing from M, or an alkaline earth metal. In US 2004/023113 A1, particular value is placed on the fact that the particle size distribution must have a defined form so that the pressure applied during the compression during electrode manufacture can spread particularly gently over the particle bed. In addition to the particle size distribution, it is also mentioned that the particles of the powder should have pores which are as small as possible and the pore volume of the pores up to a diameter of 1 μm should not exceed a value of 0.03 cm3/g (Hg porosimetry). However, no particular process engineering measures are described for achieving said product parameters. In the determination of the compressed density, the powder is compressed under a pressure of 0.3 t/cm2.
In the examples, mainly lithium cobalt oxides are described. At the abovementioned compression pressure of 0.3 t/cm3, compressed densities in the range of 2.58-3.32 g/cm3 are reached.
In addition to the compressed density itself, value is furthermore placed on the fact that, after the compression of the material, the volume fraction of the particles smaller than 1 μm is not greater than 0.1%. A significant increase in the fine particles after the compression would indicate that particles are destroyed during the application of pressure. Such a phenomenon would endanger the homogeneity of the electrode.
It is however to be assumed that a pressure of 0.3 t/cm2 does not correspond to the pressures which are actually applied during the electrode manufacture. During the electrode manufacture, the material must be built to withstand at least a pressure of 1 t/cm2. In JP 2001-80920 A, a pressure of 2 t/cm2 is stated in example 1 for the electrode manufacture. JP 2001-80920 A mentions the compressive strength of lithium mixed metal oxides (LNCO), which comprise three metallic components in addition to lithium.
The materials thus produced have a compressive strength of 0.001-0.01 N. According to this document, it is desirable for the particles to disintegrate into their primary constituents during the electrode manufacture, which is contrary to the argumentation of US 2004/023113 A1. According to JP 2001-80920 A the material which has disintegrated into smaller constituents must have a certain flowability to enable the particles to be distributed uniformly over the electrode.
The compressive strength of lithium mixed metal oxides is also discussed in US 2005/0220700 A1. There, the compounds have the formula LipNixCoyMnzMqO2-aFa. Whereas US 2004/023113 A1 only states the value 0.3 t/cm2 for the compressive strength, compressive strengths of at least 50 MPa are stated in US 2005/0220700 A1 for the lithium mixed metal compounds, which corresponds to 0.5 t/cm2. However, the formula for the relevant compounds in US 2005/0220700 A1 is defined substantially more narrowly than that in US 2004/023113 A1. Thus, manganese is a fixed constituent of all compounds in US 2005/0220700 A1.